Chrome plating of titanium

ABSTRACT

AN ADHERENT LAYER OF WEAR-RESISTANT HARD CHROMIUM IS APPLIED TO A TITANIUM OR TITANIUM ALLOY PART BY FIRST ELECTROPLATING A LAYER OF CHROMIUM ON SUCH PART, THEN SUBJECTING THE PART TO HEAT TREATMENT AT A TEMPERATURE FROM ABOUT 1600*F. TO ABOUT 1900*F. FOR A VERY BRIEF TIME (LESS THAN ONE MINUTE), TO CAUSE DIFFUSION BONDING OF THE CHROMIUM TO THE TITANIUM SUBSTRATE, AND THEN ELECTROPLATING A RELATIVELY THICK LAYER OF HARD CHROMIUM ON THE FIRST CHROMIUM LAYER. THE RESULTING HARD CHROMIUM OUTER LAYER IS FIRMLY ADHERENT, AND THE PROCEDURE AVOIDS DISTORTION OF THE TITANIUM AND MINIMIZES DETERIORATION OF ITS PHYSICAL PROPERTIES. THE FATIGUE STRENGTH OF THE TITANIUM IS INCREASED BY SHOT PEENING THE INITIAL CHROMIUM LAYER AFTER DIFFUSION BONDING THEREOF TO THE TITANIUM SUBSTRATE PRIOR TO ELECTROPLATING THE OUTER CHROMIUM LAYER.

United States Patent 3,691,029 CHROME PLATING 0F TITANIUM Louis W. Raymond, Fairlield, Conn., and Mark C.

Gussack, Riverdale, N.Y., assignors to Superior Plating Company, Fairfield, Conn.

N0 Drawing. Continuation-impart of abandoned application Ser. No. 30,223, Apr. 20, 1970. This application Mar. 5, 1971, Ser. No. 121,599

Int. Cl. C23b 5/52 US. Cl. 204--37 R 18 Claims ABSTRACT OF THE DISCLOSURE An adherent layer of wear-resistant hard chromium is applied to a titanium or titanium alloy part by first electroplating a layer of chromium on such part, then subjecting the part to heat treatment at a temperature from about 1600 F. to about 1900 F. for a very brief time (less than one minute), to cause diffusion bonding of the chromium to the titanium substrate, and then electroplating a relatively thick layer of hard chromium on the first chromium layer. The resulting hard chromium outer layer is firmly adherent, and the procedure avoids distortion of the titanium and minimizes deterioration of its physical properties. The fatigue strength of the titanium is increased by shot peening the initial chromium layer after diffusion bonding thereof to the titanium substrate prior to electroplating the outer chromium layer.

RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 30,223 filed on Apr. 20, 1970 and now abandoned.

This invention relates to a method for electroplating a hard chromium layer on a titanium or titanium alloy substrate, and more particularly, the invention relates to a method for electroplating a hard chromium layer on a titanium or titanium alloy substrate by first electrodepositing a first layer of chromium on the substrate, heat treating the chrome plated substrate, and then electrodepositing a layer of hard chromium upon the first layer. The fatigue strength of the initial chromium layer and substrate will be increased by shot peening after heat treating and cooling.

The great majority of the titanium and titanium alloy structural applications has been in the aircraft industry Where the dual characteristics of light-Weight and high mechanical strength make these materials extremely useful in numerous applications. However, one basic problem which has plagued the aircraft industry in using these materials is their relatively poor wear resistance when used in applications involving frictional contact. In seeking a solution to this problem, it has been found that the wear resistance of titanium parts is greatly increased if the titanium surface is coated with hard chromium. However, the electrodeposition of a hard chromium coating onto a titanium substrate has, in the past, presented numerous difficulties.

Several methods have been suggested for coating titanium parts by electrodeposition with a hard layer so as to improve the wear resistance of the titanium. One such method is described in US. Patent No 3,471,342 to Wood. Wood increases the wear resistance of a titanium article by electroplating a layer of chromium on the titanium article, diifusion annealing the plated article at a temperature in the range of about 1600 F. to 1900 F. for a period of time up to 24 hours, and then heating the coated article in a nitrogen atmosphere to form a nitrogen stabilized alpha titanium chromium alloy surface layer. This process has the advantage that a hard surface coating is applied to the titanium article and thus the wear resistance of the titanium is greatly improved. It suffers from the disadvantage that by heat treating the titanium parts at elevated temperatures for extended periods of time, the titanium base metal properties are affected. The general effect produced by the extended heat treatment is to cause grain growth in the titanium substrate and thus a deterioration of its mechanical properties. Furthermore, prior to electroplating with chromium, titanium articles for the aircraft industry are generally precision machined to specified size having very close tolerances. By heating the precision parts at elevated temperatures for extended periods of time, the parts are subject to a substantial amount of distortion so much so that they may be rendered unusable for their intended purpose.

Another method for improving the wear resistance of titanium articles involves applying an extremely thick single coating of hard chromium on the titanium by an electroplating operation. However, the adherence of such an electroplated coating to the titanium substrate is poor.

The present invention contemplates a relatively simple and eflicient process for coating a titanium or titanium alloy substrate with an adherent layer of hard chromium and thus increasing the wear resistance of the substrate. It has been found that by first electroplating a relatively thin layer of chromium on the titanium article, heat treating the article only sufficiently to allow for diifusion bonding of the chromium to the substrate, and then electroplating a final layer of hard chromium upon the first layer, it is possible (1) to eliminate the problem of distortion of the precision titanium parts, (2) to prevent substantially any change in the mechanical properties of the titanium base material, and (3) to produce an extremely adherent hard chromium layer on the titanium which allows the titanium articles to be subject to extreme conditions of frictional contact.

Broadly stated, the process of this invention involves a method for depositing a hard chromium electroplate on a titanium or titanium alloy substrate which comprises electroplating a thin layer of chromium (which may be hard chromium) on the substrate to a thickness sufficient to permit diffusion of the chromium into the substrate. The electroplated substrate is subject to a heat treatment at a temperature in the range from about 1600 F. to about 1900 F. for a time sutficient to allow for diffusion bonding of the chromium to the substrate but brief enough .(generally less than one minute) so as not to have any substantial effect on the physical properties of the substrate. Preferably, this is done by limiting the heating as closely as possible to the interface of titanium and chromium. The heat treated part is cooled to approximately room temperature and a layer of hard chromium, generally of a thickness substantially greater than the first chromium layer, is electroplated on the first chromium layer.

Any of the commercially available titanium alloys which are made into parts used in commercial applications involving frictional contact can be coated with hard chromium according to the present process. For example, titanium alloys containing 6% aluminum and 4% vanadium, and titanium alloys containing 7% aluminum and 4% molybdenum may be so treated, as well as other commercial titanium compositions.

Depending on the physical conditions of the metal surface which is to be plated, the titanium or titanium alloy substrate can initially be subject to a honing operation to remove substantially all the rough metal burrs from the surface. To insure a substantially completely smooth surface, the substrate may be polished chemically or mechanically. The surface may then be activated by immersing the substrate in an acidic activation solution, e.g.,

a solution of glacial acetic acid and hydrofluoric acid, and anodically treating the substrate for a period of time (say one to twenty minutes) sufficient to provide an activated surface. Alternatively, the titanium substrate can be activated by soaking the metal in an acidic etching solution.

After polishing, the substrate is rinsed in water to remove substantially all traces of the acidic bath and then without allowing the substrate to dry it is placed in a suitable vessel containing a chromium plating solution. Any of the standard commercially available chromium plating solutions may be used. This initial plating solution may, for example, be of any conventional composition used for hard chrome plating or for decorative chrome plating. After placing the substrate in the plating bath, a layer of chromium is electroplated onto the wear surface of the substrate. The chromium is plated to a thickness that is at least sufficient to permit formation of a diffusion alloy of chromium with the titanium substrate when the plated substrate is heat treated. The specific thickness of this chromium layer can vary depending upon the total thickness of chromium which is to be plated. Preferably, an initial chromium layer having a thickness ranging from about .0001" to about .008 is used. The specific current density and plating time used in the process will vary, in accordance with conventional practice. For example, variables such as the dimensions of the titanium substrate and the desired thickness of the chromium layer will determine both the current density and the plating time which is used.

After the substrate is coated with the first layer of chromium it is placed in a furnace which is capable of heating the substrate at the interface with the chromium to temperatures above 1600 F. One furnace which is particularly suitable for use in the present invention is an induction furnace. This is because, in an induction furnace the coated substrate can be heated to the required temperature in a very short period of time and the heating can be confined to the area which was plated with the chromium. Induction heating can also be limited rather closely to the interface between chromium and titanium, without penetrating deeply into the body of a titanium part of substantial mass.

When using an induction furnace, the substrate is placed in or adjacent to an induction coil which conforms to the chrome plated surface of the substrate. The substrate is then heated by the induction coil to temperatures ranging from about 1600 F. to about 1900 F., preferably in the range of about 1600 F. to about 1700 F., for periods of time ranging from about 0.01 to seconds. The specific time which the plated substrate is held at the temperature should be sufficient to permit diffusion of chromium at the interface between the titanium substrate and the electrodeposited chromium layer, but should not be longer. The heat treatment should avoid heating the titanium substrate so extensively that any substantial distortion or grain growth occurs in the substrate.

While induction heating is preferred, any other desired method of bringing the titanium at its interface with the chromium plate to the specified temperatures may be used.

The heat treatment may be performed in an inert atmosphere to avoid the formation of chromium oxide on the plated surface. Any of the commercially available inert gases may be used for this purpose, e.g. helium, argon, etc. After the substrate has been heat treated, it should be cooled to room temperature, preferably by quenching in water. This will prevent the formation of chromium oxide on the surface. However, if any chromium oxide forms as a result of the heat treatment, or slow cooling in air, it can be removed by subjecting the plated surface to a fine polishing operation. Any buffing compound, grease, etc. on the plated substrate as a result of the polishing operatlon can be removed by soaking the substrate in an alkaline cleaner, in accordance with conventional practice.

The chrome plated substrate is then desirably subjected to an activation operation. This is preferably done by anodically treating the substrate in a chrome plating solution for a time sufiicient to produce an activated chrome surface. Subsequent to this, a layer of hard chromium 1s deposited upon the first layer. This can be accomplished by switching the leads of the plating apparatus and adjusting the current so that a hard chrome coating is deposited upon the first chrome coating. The plating operation is continued until such time as the desired chromium thickness is obtained. I

The second chromium electroplate 1s deposited from any standard hard chromium plating bath, under conventional plating conditions. The final plate has the full hardness and wear resistance of conventional hard chromium, since it is not subject to softening or annealmg by heat treatment, as in the initial layer. It adheres te nac1ously to the initial layer, however, and through the initial layer is securely bonded to the titanium substrate.

As a further aspect of the invention it has been found that the fatigue strength of the titanium or titanium alloy base metal will be increased by subjecting the base metal and the initial thin chromium layer after heat treating, to shot peening in a manner which will be described. The effect of shot peening is conventionally measured In terms of the depth of compression (in inches) which results from shot peening a surface, rather than in terms of compressive stress. The depth of compression is stated in standard terms as that calculated with respect to a standardized steel specimen called an Almen Strip. Thus, the term 6A2 means the velocity needed to effect a compressive stress (in terms of depth of compression) 0.006" deep as applied to an A Almen Strip when measured by a No. 2 Almen gauge. By means of the shot peening step, a compressive stress is induced into the thin chromium precoat and into the titanium substrate which will restore and sometimes increase the titaniums original fatigue strength. Where worn titanium parts are to be electroplated in accordance with any of the above stated process, it has been found useful to apply with the initial chromium layer, an additional thickness of chromium plating to build-up worn areas which can be machined to desired dimensions, thus in effect replacing the worn portions of the titanium substrate with chrome plating. Furthermore, it has been found with respect to hard metals generally, for example: titanium, titanium alloys and steels having a Rockwell C hardness of 40 or greater than 40, that shot peening after electroplating a chromium layer thereon will substantially increase the fatigue strength of such metal reduced by prior treatment.

The invention will be described in detail with reference to the following examples for illustrative purposes.

Example I A sample substrate of a titanium alloy containing 6% aluminum and 4% vanadium was treated as follows; the sample was immersed in a bath consisting of 87.5% by volume glacial acetic acid and 12.5% by volume of hydrofluoric acid (60-70%) and anodically treated for about 1 minute at 20 amps/sq. ft. The temperature of the bath ranged from 120 F. A lead cathode was used with an anode to cathode distance of approximately of an inch. However, a cathode consisting of stainless steel could also be used. After the anodic treatment, the sample was rinsed in water and then placed directly in a conventional hard chrome plating bath with the current turned on without allowing the sample to dry. The plating bath consisted of chromic acid, sulfuric acid and water. The concentration of the chrome solution was 32 ounces per gallon and the sulphate ratio to l. The plating bath was kept at a temperature of approximately F. and the sample was plated for approximately one hour at a current density ranging from 3 to 3.5 amps/sq. in. until a chrome deposit having a thickness of approximately one 11111 was formed. After the initial plating operation,

the sample was transferred to an induction furnace. An induction coil which conformed in shape to the area of the substrate which was plated with chromium was used,

so that the surface of the substrate could be brought to the required temperature in 2. to seconds. The chrome plated sample was induction heated at 1600 F. and as soon as the chrome-titanium interface of the sample reached this temperature (approximately 4 seconds) the induction power was shut off and the sample was immediately water quenched. The brief heating time was enough to effect interdiffusion of chromium and titanium at their interface, but did not result in substantial annealing, or distortion of the main body of titanium. Although not necessary, the water quenching prevented the sample from remaining at elevated temperatures for too long a period of time. The sample was polished on a fine polishing wheel to remove any chromium oxide which formed during the induction heating. The sample was soaked in an alkaline cleaner so as to remove any remaining buffing compound and/or grease. The cleaned sample was again transferred to the same chrome plating bath used previously and anodically treated for approxiinately from 20 to 25 seconds at 3 to 3.5 amps/sq. in. Immediately after, the leads were switched and the current brought up slowly (approximately 45 seconds) to about 3.2 amps/sq. in The sample was plated to a total chromium thickness of two mils The resulting chromium plated product was subject to deformation by identation and no spalling or flaking of the chrome deposit occurred. The chromium coating had the full hardness and wear resistance of conventional hard chromium. Samples prepared as described have been subject to both an outside bend test and an inside bend test over a V. inch radius. The chrome coating remained firmly adherent to the titanium substrate throughout both bond tests. A further shot peening test showed that the two mil chromium coating remained firmly adherent to the titanium.

Example II A titanium alloy sample containing 6% aluminum and 4% vanadium was coated in two stages with a two mil coating of chromium, following in detail the same procedure as described in Example I, except that the heat treatment operation was omitted. The chromium coated sample was subject to an inside bend test around a inch radius (a test which is less severe than the /2 inch radius bend test of Example I). The bending stress caused the chrome coating to be peeled away from the titanium base.

Example 111 A strip sample of titanium alloy containing 7% aluminum and 4% molybdenum was treated as follows. The sample was immersed in a bath consisting of phosphoric acid (45 by volume), sulfuric acid (45% by volume) and hydrofluoric acid 60-70% by volume) for about 7 minutes. The temperature of the bath ranged from 110 to 140 F. The sample was then placed directly in a chrome plating bath of the same formula as in Example I, with the current turned on without allowing the sample to dry. The plating bath was kept at a temperature of approximately 140 F. and the sample was plated for approximately one hour at a current density ranging from 3 to 3.5 amps/sq. in. to produce a chrome deposit of approximately one mil. After the initial plating operation, the sample was transferred to an induction furnace. An induction coil which conformed to the plated area was used such that the surface of the sample could be brought to the required temperature in 2 to 5 seconds. The chrome plated substrate was induction heated at 1750 F. in an inert atmosphere of argon, and as soon as the sample reached this temperature (approximately 4 seconds) the induction power was shut off and the sample was immediately water quenched. The sample was substantially free from any chromium oxide on the surface. The heat treated substrate was again transferred to the chrome plating bath and anodically treated for approximately 20 to 25 seconds at a current ranging from 3 to 3.5 amps/sq. in. Immediately after the leads were switched and the current brought up slowly (approximately 45 seconds) to about 3 to 3.5 amps/sq. in., the sample was plated to a total chromium thickness of two mils. The chromium plated sample was tested by being indented and no spalling or flaking of the chrome deposit occurred. The hardness of the chrome was fully equal to conventional hard chromium. The sample was also subject to both an outside bend test and an inside bend test over a /2 inch radius. The chrome coating remained firmly adherent to the titanium sample throughout both bend tests. A further shot peening test showed that the two mil chromium coating remained firmly adherent to the titanium.

Example IV Chrome plated samples were produced in accordance with Example I and in accordance with Example II. The titanium samples were then mechanically tested to see if the heat treatment (Example I treatment) which is used in the present invention produced any adverse effects on the properties of the titanium alloy. The results are shown in Table I below. It can be seen that the listed properties of the titanium alloy base metal were not substantially affected by the short heat treatment; although fatigue strength was somewhat reduced without the shot peening step of Example VI.

Two titanium sample plates measuring 1" x 4" x .050" were coated with an initial layer of chromium by the procedure described in Example I. One of the samples was then plated in a furnace and heated to 1600 F. for five minutes. The second sample was subject to an induction heating process as described in Example I at 1600 F. for five seconds. After the heating operation, the samples were removed from the furnace and measured to see if any distortion occurred. The sample which was heated at 1600 F. for five minutes showed a deflection of about The sample subjected to the induction heating process for five seconds in accordance with the present invention showed no measurable deflection whatsoever.

Example VI Chromium plated samples are produced in accordance witn any of the preceding examples except for the following additional step. After the titanium or titanium alloy has received the initial chromium coat, has been heat treated to cause diffusion of the chromium into the titanium substrate, and has been cooled, the chromium and substrate layer are subjected to shot peening. For this purpose a steel shot of the 5-70 size (0.0165") was chosen and the velocity of the shot was that required to produce a value of 6A2 with respect to an Almen Strip. Beneficial peening effects can be observed using shot of any size with intensities of 3A2 to 12A2. The result of shot peening was to restore and in some cases increase the original fatigue strength of the titanium or titanium alloy.

After shot peening, a second layer of hard chromium is electroplated on to the shot peened layer of chromium as stated in accordance with the preceding examples. It was generally observed that metals such as titanium, titanium alloys and steels having a Rockwell C hardness of 40 or above whose respective fatigue strengths had been reduced during treatment would have such fatigue strengths increased after chrome plating by the aforementioned shot peening of the chrome layer.

Example VII A sample of 6-4 titanium (simulating a worn titanium part) was chrome-plated per Example I, except that the initial chromium layer applied was 0.025 thick. The sample was heat treated and cooled in accordance with the teaching of Example I, and after cooling, the part was machined to reduce the thickness of the softened chromium layer to about 0.012" thickness that is to say the machining produced a uniform outer dimension to a given prescribed specification. After machining, the second outer layer of hard chromium was electroplated to bring the sample to a prescribed finished outer dimension. By this means worn titanium parts can be brought back to original dimensional tolerances during the chrome plating process of the invention.

What is claimed is:

1. A method for depositing a hard chromium electroplate on a titanium or titanium alloy substrate comprising:

(a) electroplating a layer of chromium on the substrate to a thickness sufficient to permit formation of a diffusion alloy of the chromium with the substrate when the plated substrate is heat treated,

(b) subjecting the electroplated substrate to a heat treatment at a temperature in the range of about 1600 F. to about 1900 F. for a time, generally less than one minute, sufiicient to allow for diffusion bonding of the chromium to the substrate,

(c) cooling the substrate to approximately room temperature, and

(d) electroplating on the first chromium layer a layer of hard chromium.

2. The method according to claim 1 in which the second layer of hard chromium has a thickness greater than the first chromium layer.

3. The method according to claim 1 in which the substrate is subject to an anodic pre-treatment prior to step (a).

4. The method according to claim 1 in which the heat treatment of step (b) is performed in an induction furnace' for a period of time ranging from about 0.01 to 10 seconds.

5. The method according to claim 1 in which steps (b) and (c) are performed in an inert atmosphere.

6. The method according to claim 1 in which the thickness of the chromium layer of step (a) ranges from about .0001" to about .008".

7. The method according to claim 1 in which the substrate is cooled to room temperature by water quenching.

8. The method according to claim 1 in which the titanium and titanium alloy substrate has been worn by previous use, the layer of chromium deposited during step (a) is greater than the thickness of material worn, and after steps (b) and (e) have been performed the initial chromium layer is machined to prescribed outer dimensions.

9. The method according to claim 1 in which after heat treatment of the electroplated substrate and cooling thereof, the initial chromium layer and substrate are subjected to shot peening to increase the fatigue strength of said layer and substrate.

10. The method according to claim 9 wherein said shot peening is performed with an intensity within the range of 3A2 to 12A2.

11. The method according to claim 10 wherein the intensity of shot peening is about 6A2 using S- steel shot.

12. The method according to claim 8 in which after machining of said initial chromium layer, said layer is subjected to shot peening to increase the fatigue strength of said layer and substrate.

13. A method for depositing a hard chromium electroplate on a titanium or titanium alloy substrate comprismg:

(a) subjecting the substrate to an anodic treatment in an acid bath,

(b) electroplating a layer of chromium on the substrate to a thickness of at least about 0.0001",

(c) subjecting the plated surface to a heat treatment in an induction furnace under an inert atmosphere at a temperature in the range of about 1600" F. to about 1900 F. for a time sufficient to allow for diffusion bonding of the chromium to the substrate, but limiting such heating to a time less than one minute and as closely as possible to the interface of the chromium and titanium so as to minimize the effect of such heating on the physical properties of the substrate,

(d) cooling the substrate to approximately room temperature, and

(e) electroplating on the first chromium layer a second layer of chromium.

14. The method according to claim 13 in which the heat treatment of step (c) is performed at a temperature in the range of about 1600 F. to about 1750 F.

15. The method according to claim 13 in which the titanium and titanium alloy substrate has been worn by previous use, the layer of chromium deposited during step (b) is greater than the thickness of material worn, and after steps (c) and (d) have been performed, the initial chromium layer is machined to prescribed outer dimen- 16. The method according to claim 13 in which after heat treatment of the electroplated substrate and cooling thereof, the initial chromium layer and substrate are subjected to shot peening to increase the fatigue strength of said layer and substrate.

17. The method according to claim 16 wherein said shot peening is performed with an intensity within the range of 3A2 to 12A2.

18. The method according to claim 17 wherein the intensity of shot peening is about 6A2 using S-70 steel shot.

References Cited UNITED STATES PATENTS 1,845,978 2/1932 Hosenfeld 204-39 3,471,342 10/1969 Wood 204-37 R 2,786,809 3/1957 Raynes 204--39 FOREIGN PATENTS 788,804 1/1958 Great Britain 204-37 R JOHN H. MACK, Primary Examiner W. I. SOLOMON, Assistant Examiner U.S. Cl. X.R. 

